Bohr model—The model of the hydrogen atom in which electrons assume certain circular orbits around a positive nucleus.

Boiling point—The temperature at which the vapor pressure of a liquid is equal to the incident pressure; the normal boiling point of any liquid is defined as its boiling point at a pressure of 1 atmosphere.

Boiling point elevation—The amount by which a given quantity of solute raises the boiling point of a liquid; a colligative property.

Bond length—The average distance between two nuclei in a bond; as the number of shared electron pairs increases, the bond length decreases.

Bond order—The number of shared electron pairs between two atoms; a single bond has a bond order of 1, a double bond has a bond order of 2, a triple bond has a bond order of 3.

Bonding orbital—A molecular orbital formed by the overlap of two or more atomic orbitals; energy is less than that of the combining orbitals.

Boyle’s law—The law stating that at constant temperature, the volume of a gaseous sample is inversely proportional to its pressure.

Broken-order reaction—A reaction with noninteger orders in its rate law.

Brønsted–Lowry acid—A proton donor.

Brønsted–Lowry base—A proton acceptor.

Buffer—A solution containing a weak acid and its salt (or a weak base and its salt) that tends to resist changes in pH.

Buffer region—The portion of a titration curve in which the concentration of an acid is approximately equal to that of its conjugate base; pH remains relatively constant through this region.

Calorie (cal)—A unit of thermal energy.

Calorimeter—An apparatus used to measure the heat absorbed or released by a reaction.

Catalyst—A substance that increases the rates of the forward and reverse directions of a specific reaction by lowering activation energy, but is itself left unchanged.

Cathode—The electrode at which reduction takes place.

Cation—An ionic species with a positive charge.

Celsius (°C)—A temperature scale defined by having 0°C equal to the freezing point of water and 100°C equal to the boiling point of water; otherwise known as the centigrade temperature scale.

Chalcogens—Elements found in Group VIA of the Periodic Table with diverse chemistry; the group contains metals, nonmetals (like oxygen), and metalloids; typically form –2 anions.

Charging—A state of an electrochemical cell in which an external electromotive force is being used to return a cell to its original state; during this process, electrons are transferred nonspontaneously from cathode to anode.

Charles’s law—The law stating that the volume of a gas at constant pressure is directly proportional to its absolute (kelvin) temperature.

Chelation—The process of binding metal ions to the same ligand at multiple points.

Chemical bond—The interaction between two atoms resulting from the sharing or transfer of electrons.

Chemical equation—An expression used to describe the quantity and identity of the reactants and products of a reaction.

Chemical properties—Those properties of a substance related to the chemical changes that it undergoes, such as ionization energy and electronegativity.

Closed system—A system that can exchange energy but not matter with its surroundings.

Colligative properties—Those properties of solutions that depend only on the number of solute particles present but not on the nature of those particles.

Collision theory of chemical kinetics—A theory that states that the rate of a reaction is proportional to the number of collisions per second between reacting molecules that have sufficient energy to overcome the activation energy barrier; implies that only a fraction of collisions are sufficient.

Combination reaction—A reaction in which two or more reactants form a single product.

Combined gas law—A gas law that combines Boyle’s law, Charles’s law, and Gay-Lussac’s law to state that pressure and volume are inversely proportional to each other, and each is directly proportional to temperature.

Combustion reaction—A reaction in which an oxidant (typically oxygen) reacts with a fuel (typically a hydrocarbon) to yield water and an oxide (such as carbon dioxide if between a hydrocarbon and oxygen).

Common ion effect—A shift in the equilibrium of a solution due to the addition of ions of a species already present in the reaction mixture.

Complex ion—A polyatomic molecule in which a central cation is bonded to electron pair donors called ligands.

Compound—A pure substance that can be decomposed to produce elements, other compounds, or both.

Compression—Reduction in the volume of a gas.

Concentrated solution—A solution with a high concentration value; the cutoff for the term “concentrated” depends on the purpose and identity of the solution.

Concentration—The amount of solute per unit of solvent or the relative amount of one component in a mixture.

Concentration cell—A cell that creates an electromotive force (emf or voltage) using a single chemical species in half-cells of varying concentration.

Condensation—The process in which a gas transitions to the liquid state.

Conductor—A material in which electrons are able to transfer energy in the form of heat or electricity.

Conjugate acid–base pair—The relationship between a Brønsted–Lowry acid and its deprotonated form, or a Brønsted–Lowry base and its protonated form.

Coordinate covalent bond—A covalent bond in which both electrons of the bonding pair are donated by one of the bonded atoms.

Covalent bond—A chemical bond formed by the sharing of an electron pair between two atoms; can be in the form of single bonds, double bonds, or triple bonds.

Critical pressure—The vapor pressure at the critical temperature of a given substance.

Critical temperature—Also known as the critical point. The highest temperature at which the liquid and gas phases of a substance can coexist; above this temperature, the liquid and gas phases are indistinguishable.

Crystal—A solid in which atoms, ions, or molecules are arranged in a regular, three-dimensional lattice structure.

d subshell—Subshell corresponding to the angular momentum quantum number l = 2; contains five orbitals and is found in the third and higher principal energy levels.

Dalton’s law of partial pressures—The law stating that the sum of the partial pressures of the components of a gaseous mixture must equal the total pressure of the sample.

Daniell cell—An electrochemical cell in which the anode is the site of Zn metal oxidation and the cathode is the site of Cu2+ ion reduction.

Decomposition reaction—A reaction in which a single compound breaks down into two or more products.

Delocalized orbitals—Molecular orbitals in which electron density is spread over an entire molecule, or a portion thereof, rather than being localized between two atoms.

Density (ρ)—A physical property of a substance, defined as the mass contained in a unit of volume.

Deposition—In most chemical processes, the direct transition of a substance from the gaseous state to the solid state; in electrochemical reactions, the build up of a solid precipitate onto an electrode.

Diamagnetism—A condition that arises when a substance has no unpaired electrons and is slightly repelled by a magnetic field.

Diffusion—The random motion of gas or solute particles across a concentration gradient, leading to uniform distribution of the gas or solute throughout the container.

Dilute solution—A solution with a low concentration of a given solute.

Dipole—A species containing bonds between elements of different electronegativities, resulting in an unequal distribution of charge.

Dipole–dipole interactions—The attractive forces between two dipoles; magnitude is dependent on both the dipole moments and the distance between the two species.

Dipole moment—A vector quantity with a magnitude that is dependent on the product of the charges and the distance between them; oriented from the positive to the negative pole.

Discharging—The state of a rechargeable electrochemical cell that is providing an electromotive force by allowing electrons to flow spontaneously from anode to cathode.

Disproportionation—An oxidation–reduction reaction in which the same species acts as the oxidizing agent and as the reducing agent; also called dismutation.

Dissociation—The separation of a single species into two separate species; usually used in reference to salts or weak acids or bases.

Double-displacement reaction—A reaction in which ions from two different compounds swap their associated counterions; typically, one of the products of this type of reaction is insoluble in solution and will precipitate.

Ductility—The property of metals that allows a material to be drawn into thinly stretched wires.

Effective nuclear charge (Zeff)—The charge perceived by an electron from the nucleus; applies most often to valence electrons and influences periodic trends such as atomic radius and ionization energy.

Effusion—The movement of gas from one compartment to another under pressure through a small opening; follows Graham’s law.

Electrochemical cell—A cell within which an oxidation–reduction reaction takes place, containing two electrodes between which there is an electrical potential difference.

Electrode—An electrical conductor through which an electrical current enters or leaves a medium.

Electrolysis—The process in which an electrical current is used to power an otherwise nonspontaneous decomposition reaction.

Electrolyte—A compound that ionizes in water and increases the conductance of the solution.

Electromagnetic radiation—A wave composed of electric and magnetic fields oscillating perpendicular to each other and to the direction of propagation.

Electromagnetic spectrum—The range of all possible frequencies or wavelengths of electromagnetic radiation.

Electromotive force (emf)—The potential difference developed between the cathode and the anode of an electrochemical cell; also called voltage.

Electron (e–)—A subatomic particle that remains outside the nucleus and carries a single negative charge; in most cases, its mass is considered to be negligible.

Electron configuration—The symbolic representation used to describe the electron arrangement within the energy sublevels in a given atom.

Electron spin—The intrinsic angular momentum of an electron, represented by ms; has arbitrary values of and .

Electronegativity—A measure of the ability of an atom to attract the electrons in a bond; commonly measured with the Pauling scale.

Element—A substance that cannot be further broken down by chemical means; defined by its number of protons (atomic number).

Emission spectrum—A series of discrete lines at characteristic frequencies, each representing the energy emitted when electrons in an atom return from an excited state to their ground state.

Empirical formula—The simplest whole-number ratio of the different elements in a compound.

Endothermic reaction—A reaction that absorbs heat from the surroundings as the reaction proceeds (positive ΔH).

Endpoint—The point in a titration at which the indicator changes to its final color.

Energy density—An equivalence unit regarding the amount of electrochemical energy capable of being stored per unit weight; a battery with a large energy density can produce a large amount of energy with a small amount of material.

Enthalpy (H)—The heat content of a system at constant pressure; the change in enthalpy (ΔH) in the course of a reaction is the difference between the enthalpies of the products and the reactants.

Entropy (S)—A property related to dispersion of energy through a system or the degree of disorder in that system; the change in entropy (ΔS) in the course of a reaction is the difference between the entropies of the products and the reactants.

Equilibrium—The state of balance in which the forward and reverse reaction rates of a reversible reaction are equal; the concentrations of all species will remain constant over time unless there is a change in the reaction conditions.

Equilibrium constant (Keq)—The ratio of the concentrations of the products to the concentrations of the reactants for a certain reaction at equilibrium, all raised to their stoichiometric coefficients.

Equivalence point—The point in a titration at which the moles of acid present equal the moles of base added, or vice-versa.

Equivalent—A mole of charge in the form of electrons, protons, ions, or other measurable quantities that are produced by a substance.

Evaporation—The transition from a liquid to a gaseous state.

Excess reagent—In a chemical reaction, any reagent that does not limit the amount of product that can be formed.

Excitation—The promotion of an electron to a higher energy level by absorption of an energy quantum.

Excited state—An electronic state having a higher energy than the ground state; typically attained by the absorption of a photon of a certain energy.

Exothermic reaction—A reaction that gives off heat to the surroundings (negative ΔH) as the reaction proceeds.

f subshell—The subshell corresponding to the angular momentum quantum number l = 3; contains seven orbitals and is found in the fourth and higher principal energy levels.

Faraday constant (F)—The total charge on 1 mole of electrons ; not to be confused with the farad (also denoted F), a unit of capacitance.

First law of thermodynamics—The law stating that the total energy of a system and its surroundings remains constant.

First-order reaction—A reaction in which the rate is directly proportional to the concentration of only one reactant.

Fluid—A substance that flows due to weak intermolecular attractions between molecules and that takes the shape of its container; liquids and gases are considered fluids.

Formal charge—The conventional assignment of charges to individual atoms of a Lewis structure for a molecule; the total number of valence electrons in the free atom minus the total number of electrons when the atom is bonded (assuming equal splitting of the electrons in bonds).

Formula weight—The sum of the atomic weights of constituent ions according an ionic compound’s empirical formula.

Freezing—The process in which a liquid transitions to the solid state; also known as solidification or crystallization.

Freezing point—At a given pressure, the temperature at which the solid and liquid phases of a substance coexist in equilibrium; identical to the melting point.

Freezing point depression—Amount by which a given quantity of solute lowers the freezing point of a liquid; a colligative property.

Galvanic cell—An electrochemical cell that uses a spontaneous oxidation–reduction reaction to generate an electromotive force; also called a voltaic cell.

Gas—The physical state of matter possessing the most disorder, in which molecules interact through very weak attractions; found at relatively low pressure and high temperatures.

Gas constant (R)—A proportionality constant that appears in the ideal gas law equation, PV = nRT. Its value depends on the units of pressure, temperature, and volume used in a given situation.

Gay-Lussac’s law—The law stating that the pressure of a gaseous sample at constant volume is directly proportional to its absolute temperature.

Gibbs free energy (G)—The energy of a system available to do work. The change in Gibbs free energy, ΔG, can be determined for a given reaction equation from the enthalpy change, temperature, and entropy change; a negative ΔG denotes a spontaneous reaction, while a positive ΔGdenotes a nonspontaneous reaction.

Graham’s law—The law stating that the rate of effusion or diffusion for a gas is inversely proportional to the square root of the gas’s molar mass.

Gram equivalent weight (GEW)—The amount of a compound that contains 1 mole of reacting capacity when fully dissociated; one GEW equals the molar mass divided by the reactive capacity (how many of the species of interest is obtained) per formula unit.

Ground state—The unexcited state of an electron.

Group—A vertical column of the Periodic Table containing elements that are similar in their chemical properties; also called a family.

Half-cell—The separated compartments housing the electrodes and solutions in an electrochemical reaction.

Half-reaction—Either the reduction half or oxidation half of an oxidation–reduction reaction; in an electrochemical cell, each half-reaction occurs at one of the electrodes.

Halogens—The active nonmetals in Group VIIA of the Periodic Table, which have high electronegativities and high electron affinities.

Heat—The energy transferred spontaneously from a warmer sample to a cooler sample.

Heat of formation (ΔHf)—The heat absorbed or released during the formation of a pure substance from its elements at a constant pressure.

Heat of fusion (ΔHfus)—The enthalpy change for the conversion of 1 gram or 1 mole of a solid to a liquid at constant temperature and pressure.

Heat of sublimation (ΔHsub)—The enthalpy change for the conversion of 1 gram or 1 mole of a solid to a gas at constant temperature and pressure.

Heat of vaporization (ΔHvap)—The enthalpy change for the conversion of 1 gram or 1 mole of a liquid to a gas at constant temperature and pressure.

Heisenberg uncertainty principle—The concept that states that it is impossible to determine both the momentum and position of an electron simultaneously with perfect accuracy.

Henderson–Hasselbalch equation—Equation showing the relationship of the pH or pOH of a solution to the pKa or pKb and the ratio of the concentrations of the dissociated species.

Henry’s law—The law stating that the mass of a gas that dissolves in a solution is directly proportional to the partial pressure of the gas above the solution.

Hess’s law—The law stating that the energy change in an overall reaction is equal to the sum of the energy changes in the individual reactions that comprise it.

Heterogeneous—Nonuniform in composition.

Heterogeneous catalyst—A catalyst that is not in the same phase of matter as the reactants (for example, a solid platinum catalyst reacting with hydrogen gas).

Homogeneous—Uniform in composition.

Homogeneous catalyst—A catalyst that is in the same phase of matter as the reactants (for example, an aqueous enzyme in the cytoplasm of a cell).

Hund’s rule—The rule that electrons will fill into separate orbitals with parallel spins before pairing within an orbital.

Hybridization—The combination of two or more atomic orbitals to form new orbitals with properties that are intermediate between those of the original orbitals.

Hydrogen bonding—The strong attraction between a hydrogen atom bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) in one molecule and a highly electronegative atom in another molecule.

Hydrolysis—A reaction in which water is consumed during the breakdown of another molecule.

Hydronium ion—The H3O+ ion.

Hydroxide ion—The OH– ion.

Ideal bond angle—An angle between nonbonding or bonding electron pairs that minimizes the repulsion between them.

Ideal gas—A hypothetical gas with behavior that is described by the ideal gas law under all conditions; assumes that its particles have zero volume and do not exhibit interactive forces.

Ideal gas law—The equation stating PV= nRT, where R is the gas constant; can be used to describe the behavior of many real gases at moderate pressures and temperatures significantly above absolute zero.

Indicator—A substance used in low concentrations during a titration that changes color over a certain pH range (acid–base titrations) or at a particular electromotive force (oxidation–reduction titrations); the final color change of an indicator occurs at the endpoint of a titration.

Inert gases—The elements in Group VIIIA, which contain a full octet of valence electrons in their outermost shells and are therefore very unreactive; also called noble gases.

Intermediate—A molecule that transiently exists in a multistep reaction; does not appear in the overall balanced equation.

Intramolecular forces—The attractive forces between atoms within a single molecule (ionic and covalent bonds).

Ion—A charged atom or molecule that results from the loss or gain of electrons.

Ion product (IP)—The general term for the reaction quotient of a dissolving ionic compound; compared to Ksp to determine the saturation status of a solution.

Ionic bond—A chemical bond formed through electrostatic interaction between positive and negative ions.

Ionic radius—The average distance from the center of the nucleus to the edge of its electron cloud; cationic radii are generally smaller than their parent metal, whereas anionic radii are generally larger than their parent nonmetal.

Ionic solid—A solid consisting of positive and negative ions arranged into crystals that are made up of regularly repeated units held together by ionic bonds.

Ionization energy—The energy required to remove an electron from the valence shell of a gaseous atom.

Isobaric process—A process that occurs at constant pressure.

Isolated system—A system that can exchange neither matter nor energy with its surroundings.

Isothermal process—A process that occurs at constant temperature.

Isotopes—Atoms containing the same number of protons but different numbers of neutrons.

Isovolumetric process—A process that occurs at constant volume in which the system performs no work; also called an isochoric process.

Joule (J)—The unit of energy; .

Kelvin (K)—A temperature scale with units equal to the units of the Celsius scale and absolute zero defined as 0 K; also called the absolute temperature scale.

Kinetic molecular theory—The theory proposed to account for the observed behavior of gases; considers gas molecules to be pointlike, volumeless particles exhibiting no intermolecular forces that are in constant random motion and undergo only completely elastic collisions with the container or other gas particles.

Kinetic product—The product of a reaction that is formed favorably at a lower temperature because thermal energy is not available to form the transition state required to create a more stable thermodynamic product; has a smaller overall difference in free energy between the products and reactants than the thermodynamic product.

Law of conservation of charge—The law stating that, in a given reaction, the charge of ions in the products is equal to the charge of ions in the reactants.

Law of conservation of mass—The law stating that, in a given reaction, the mass of the products is equal to the mass of the reactants.

Law of constant composition—The law stating that the elements in a pure compound are found in specific mass ratios.

Law of mass action—The form of the equilibrium constant; has the concentrations of products over concentrations of reactants, each raised to their stoichiometric coefficients.

Le Châtelier’s principle—The observation that when a system at equilibrium is disturbed or stressed, the system will react in such a way as to relieve the stress and restore equilibrium.

Lead–acid battery—An electrochemical cell in which the anode is the site of Pb metal oxidation and the cathode is the site of Pb4+ ion reduction. The electrolyte is a strong acid, usually sulfuric acid.

Lewis acid—A species capable of accepting an electron pair.

Lewis base—A species capable of donating an electron pair.

Lewis structure—A method of representing the shared and unshared electrons of an atom, molecule, or ion; also called a Lewis dot diagram.

Ligand—A molecule bonded to a metal ion in a coordination compound; ligands are Lewis bases that form coordinate covalent bonds with the central metal ion.

Limiting reagent—In a chemical reaction, the reactant present in such quantity as to limit the amount of product that can be formed.

Liquid—The state of matter in which intermolecular attractions are intermediate between those in gases and in solids, distinguished from the gas phase by having a definite volume and from the solid phase because molecules may mix freely.

Magnetic quantum number (ml)—The third quantum number, defining the particular orbital of a subshell in which an electron resides; conveys information about the orientation of the orbital in space.

Malleability—A physical property of metals that defines how well an element can be shaped using a hammer.

Mass—A physical property representing the amount of matter in a given sample.

Maxwell–Boltzmann distribution curve—The distribution of the molecular speeds of gas particles at a given temperature; as temperature increases, average speed increases and the distribution becomes wider and flatter.

Melting point—The temperature at which the solid and liquid phases of a substance coexist in equilibrium; identical to the freezing point.

Metal—One of a class of elements on the left side of the Periodic Table possessing low ionization energies and electronegativities; readily give up electrons to form cations and possess relatively high electrical conductivity.

Metalloid—An element possessing properties intermediate between those of a metal and those of a nonmetal; also called a semimetal.

Millimeters of mercury (mmHg)—A unit of pressure defined as the number of millimeters that mercury in a barometer is raised above its surface in a capillary tube by an external pressure; 1 torr is equal to 1 mmHg by definition, and 1 atmosphere is equal to 760 mmHg.

Mixed-order reaction—A reaction in which the reaction order changes over time in the rate law.

Molality (m)—A concentration unit equal to the number of moles of solute per kilogram of solvent.

Molar mass—The mass in grams of one mole of an element or compound.

Molarity (M)—A concentration unit equal to the number of moles of solute per liter of solution.

Mole—An amount of substance equal to Avogadro’s number of molecules or atoms; the mass of 1 mole of substance in grams is the same as the mass of one molecule or atom in atomic mass units.

Mole fraction (X)—A unit of concentration equal to the ratio of the number of moles of a particular component to the total number of moles for all species in the system.

Molecular formula—A formula showing the actual number and identity of all atoms in each molecule of a compound; always a whole-number multiple of the empirical formula.

Molecular orbital—The region of electron density in chemical bonding that results from the overlap of two or more atomic orbitals.

Molecular weight—The sum of the atomic weights of all the atoms in a molecule.

Molecule—The smallest polyatomic unit of an element or compound that exists with distinct chemical and physical properties.

Nernst equation—An equation that relates the voltage of an electrochemical cell to the concentrations of the reactants and products within that cell.

Net ionic equation—A reaction equation showing only the species actually participating in the reaction.

Neutral solution—An aqueous solution in which the concentration of H+ and OH– ions are equal (pH = 7 at 298 K).

Neutralization reaction—A reaction between an acid and base in which a salt is formed (and sometimes water).

Neutron—A subatomic particle contained within the nucleus of an atom; carries no charge and has a mass very slightly larger than that of a proton.

Nickel–cadmium battery—A rechargeable electrochemical cell in which the anode is the site of Cd metal oxidation and the cathode is the site of Ni2+ ion reduction.

Nickel–metal hydride battery—A rechargeable electrochemical cell in which the anode is the site of metal hydride oxidation and the cathode is the site of nickel ion reduction; the nickel may be in one of many oxidation states.

Nonelectrolyte—A compound that does not ionize in water.

Nonmetal—One of a class of elements with high ionization energies and electron affinities that generally gain electrons to form anions; located in the upper right corner of the Periodic Table.

Nonrepresentative element—Elements with an expanded valence shell that includes d- and f-block electrons; also called Group B or transition elements.

Nonspontaneous process—A process that will not occur on its own without energy input from the surroundings; has a positive change in free energy.

Normality (N)—A concentration unit equal to the number of equivalents per liter of solution.

Nucleus—The small central region of an atom; a dense, positively charged area containing protons and neutrons.

Octet—Eight valence electrons in a subshell around a nucleus; imparts great stability to an atom.

Octet rule—A rule stating that bonded atoms tend to undergo reactions that will produce a complete octet of valence electrons; applies without exception only to C, N, O, and F.

Open system—A system that can exchange both energy and matter with its surroundings.

Orbital—A region of electron density around an atom or molecule containing no more than two electrons of opposite spin.

Osmosis—The movement of water through a semipermeable membrane down its concentration gradient, from low solute concentration to high solute concentration.

Osmotic pressure—The pressure that must be applied to a solution to prevent the passage of water through a semipermeable membrane down its concentration gradient; best thought of as a “sucking” pressure drawing water into solution.

Oxidation number—The number assigned to an atom in an ion or molecule that denotes its real or hypothetical charge, assuming that the most electronegative element in a bond is awarded all of the electrons in that bond.

Oxidation potential—The ability of a substance to be spontaneously oxidized; a more positive oxidation potential (measured in volts) is indicative of a substance that is easier to oxidize and will therefore more likely act as an anode in an electrochemical cell.

Oxidizing agent—In an oxidation–reduction reaction, the atom that facilitates the oxidation of another species; the oxidizing agent gains electrons and is thereby reduced.

p subshell—The subshell corresponding to the angular momentum quantum number l = 1; contains three dumbbell-shaped orbitals oriented perpendicular to each other (px, py, and pz) and is found in the second and higher principal energy levels.

Paired electrons—Two electrons in the same orbital with assigned spins of and .

Paramagnetism—A condition that arises when a substance has unpaired electrons and is slightly attracted to a magnetic field.

Partial pressure—The pressure that one component of a gaseous mixture would exert if it were alone in the container.

Pascal (Pa)—The SI unit for pressure, equivalent to .

Pauli exclusion principle—The principle stating that no two electrons within an atom may have an identical set of quantum numbers.

Percent composition—The percentage of the total formula weight of a compound attributable to a given element.

Percent yield—The percentage of the theoretical product yield that is actually recovered when a chemical reaction occurs; obtained by dividing the actual yield by the theoretical yield and multiplying by 100%.

Period—A horizontal row of the Periodic Table containing elements with the same number of electron shells.

Periodic law—The law stating that the chemical properties of elements depend on the atomic number of the elements and change in a periodic fashion.

Periodic Table—The visual display of all known chemical elements arranged in rows (periods) and columns (groups) according to their atomic number and electron structure.

pH—A measure of the hydrogen ion content of an aqueous solution, defined as the negative log of the H+ (H3O+) concentration.

pH meter—A device used to measure the concentration of hydrogen ions in solution and report it as a pH value.

Phase—One of the three forms of matter: solid, liquid, or gas; also called state.

Phase diagram—A plot, usually of pressure vs. temperature, showing which phases of a compound will exist under any set of conditions.

Photon—The form of light which displays particulate and quantal behavior.

Physical property—A property of a substance unrelated to its chemical behavior, such as melting point, boiling point, density, or odor.

Pi (π) bond—A bond with two parallel electron cloud densities formed between two p-orbitals that limits the possibility of free rotation; π bonds are the second bond in a double bond and both the second and third bonds in a triple bond.

pOH—A measure of the hydroxide (OH–) ion content of an aqueous solution, defined as the negative log of the OH– concentration.

Polar covalent bond—A covalent bond between atoms with different electro-negativities in which electron density is unevenly distributed, giving the bond positive and negative ends.

Polar molecule—A molecule possessing one or more polar covalent bonds and a geometry that allows the bond dipole moments to sum to a net dipole moment.

Polyvalent acid—An acid capable of donating more than one proton; also known as a polyprotic acid.

Polyvalent base—A base capable of donating more than one hydroxide or accepting more than one proton.

Potential energy diagram—A graph that shows the potential energies of the reactants and products of a reaction during the course of the reaction; by convention, the x-axis shows the progress of the reaction and the y-axis shows potential energy.

Potentiometer—A device used to measure electromotive force (voltage).

Precipitate—An insoluble solid that separates from a solution; generally the result of mixing two or more solutions or of a temperature change.

Principal quantum number (n)—The first quantum number, which defines the energy level or shell occupied by an electron.

Proton (p+)—A subatomic particle that carries a single positive charge and has a mass slightly less than 1 amu.

Quantum number—A number used to describe the energy levels in which electrons reside; all electrons in an element are described by a unique set of four quantum numbers.

Radioactivity—A phenomenon exhibited by certain unstable isotopes in which they undergo spontaneous nuclear transformation via emission of one or more particles.

Raoult’s law—A law stating that the partial pressure of a component in a solution is proportional to the mole fraction of that component in the solution; provides an explanation for vapor pressure depression seen in solutions.

Rate constant—The proportionality constant in the rate law of a reaction; specific to a particular reaction at a given temperature.

Rate-determining step—The slowest step of a reaction mechanism; this step serves as a bottleneck on the progress of the reaction.

Rate law—A mathematical expression giving the rate of a reaction as a function of the concentrations of the reactants; must be determined experimentally.

Rate order—The exponential effect of a change in concentration of a reactant on the change of rate in a reaction; the overall rate order is the sum of all the individual reactant rate orders.

Reaction mechanism—The series of steps that occurs in the course of a chemical reaction, often including the formation and destruction of reaction intermediates.

Reaction order—In a calculation of the rate law for a reaction, the sum of the exponents to which the concentrations of reactants must be raised.

Reaction quotient (Q)—Has the same form as the equilibrium constant, but the concentrations of products and reactants may not be at equilibrium; when compared to Keq, it dictates the direction a reaction will proceed spontaneously.

Reaction rate—The speed at which a substance is produced or consumed by a reaction.

Real gas—A gas that exhibits deviations from the ideal gas law due to molecular attractions and the actual volume of the gas molecules themselves.

Rechargeable battery—An electrochemical cell that can undergo a reversible oxidation–reduction process; when discharging, it functions as a galvanic (voltaic) cell, and when charging, it functions as an electrolytic cell.

Redox titration—A specific method used to determine the concentration of an unknown solution using reducible titrants or titrands, typically by measuring voltage changes.

Reducing agent—In an oxidation–reduction reaction, the atom that facilitates the reduction of another species; the reducing agent loses electrons and is thereby oxidized.

Reduction potential—The ability of a substance to be spontaneously reduced; a more positive reduction potential (measured in volts) is indicative of a substance that is easier to reduce and will therefore more likely act as a cathode in an electrochemical cell.

Representative elements—Elements in Groups 1, 2, and 13 through 18 in the modern IUPAC table (the s- and p-blocks of the table, also called A group elements); these elements tend to have valence shells that follow the octet rule.

Resonance—A difference in the arrangement of electron pairs but not the bond connectivity or overall charge within a Lewis structure.

Resonance hybrid—A Lewis structure that represents the weighted average (by stability) of all possible resonance structures.

Root-mean-square speed (urms)—The average speed of a gas molecule at a given temperature; as a scalar, it does not take direction into account.

s subshell—Subshell corresponding to the angular momentum quantum number l = 0 and containing one spherical orbital; found in all energy levels.

Salt—An ionic substance consisting of cations and anions.

Salt bridge—A component of an electrochemical cell composed of an inert electrolyte that allows the charge gradient that builds up in the half-cells to be dissipated as a reaction occurs; contains ions that will not react with electrodes or ions in solution and that can move to balance charge.

Saturated solution—A solution containing the maximum amount of solute that can be dissolved in a particular solvent at a given temperature.

Second law of thermodynamics—The law stating that all spontaneous processes lead to an increase in the entropy of the universe.

Second-order reaction—A reaction in which the rate is directly proportional to the concentration of two reactants, or to the square of one single reactant.

Semipermeable—A quality of a membrane allowing only some components of a solution to pass through, usually including the solvent, while limiting the passage of other species.

Sigma (σ) bond—A head-to-head bond between two orbitals of different atoms that allows free rotation about its axis.

Single-displacement reaction—A reaction in which an ion of one compound is replaced with another ion; also known as a single-replacement reaction.

Solid—The phase of matter possessing the greatest order; molecules are fixed in a rigid structure.

Solubility—A measure of the amount of solute that can be dissolved in a solvent at a certain temperature.

Standard hydrogen electrode (SHE)—The electrode defined as having a potential of zero under standard conditions; all oxidation and reduction potentials are measured relative to the standard hydrogen electrode at 25°C and with 1 M concentrations of each ion in solution.

Standard potential—The voltage associated with a half-reaction of a specific oxidation–reduction reaction; generally tabulated as reduction potentials, compared to the standard hydrogen electrode.

Standard state—The phase of matter for a certain element under standard conditions.

Standard temperature and pressure (STP)—Defined as 0°C (273 K) and 1atm; used for measuring characteristics of an ideal gas.

State function—A function that depends on the state of a system but not on the path used to arrive at that state; includes pressure, density, temperature, volume, enthalpy, internal energy, Gibbs free energy, and entropy.

Stoichiometry—A form of dimensional analysis focusing on the relationships between amounts of reactants and products in a reaction.

Sublimation—A change of phase from solid to gas without passing through the liquid phase.

Subshell—The division of electron shells or energy levels into different values of the azimuthal quantum number (s, p, d, and f); composed of orbitals.

Surroundings—All matter and energy in the universe not included in the particular system under consideration.

System—The matter and energy under consideration.

Temperature—A measure of the average kinetic energy of the particles in a system.

Theoretical yield—The maximal amount of product that can be obtained in a reaction; determined by stoichiometric analysis of the limiting reagent.

Thermodynamic product—The product of a reaction that is formed favorably at a higher temperature because thermal energy is available to form the transition state of the more stable product; has a larger overall difference in free energy between the products and reactants than the kinetic product.

Titrand—A solution of unknown concentration to which a solution of known concentration is added to determine its concentration.

Titrant—A solution of known concentration that is slowly added to a solution of unknown concentration to determine its concentration.

Titration—A method used to determine the concentration of an unknown solution by gradual reaction with a solution of known concentration.

Titration curve—A plot of the pH of a solution vs. the volume of acid or base added in an acid–base titration, or a plot of the electromotive force of a solution vs. the volume of oxidizing or reducing agent added in an oxidation–reduction titration.

Transition metal—Any of the elements in the B groups of the Periodic Table, all of which have partially filled d subshells.

Transition state—The point during a reaction in which old bonds are partially broken and new bonds are partially formed; has a higher energy than the reactants or products of the reaction and is also called the activated complex.

Triple point—The pressure and temperature at which the solid, liquid, and gas phases of a particular substance coexist in equilibrium.

Unsaturated solution—A solution into which more solute may be dissolved before reaching saturation.

Valence electron—An electron in the highest occupied energy level of an atom; the tendency of a given valence electron to be retained or lost determines the chemical properties of an element.

Valence shell electron pair repulsion (VSEPR) theory—A system that reflects the geometric arrangement of a molecule based on its Lewis dot structure; the three-dimensional structure is determined by the repulsions between bonding and nonbonding electron pairs in the valence shells of atoms.

van der Waals equation of state—One of several real gas laws, which corrects for attractive forces and the volumes of gas particles, which are assumed to be negligible in the ideal gas law.

Vapor pressure—The partial pressure of a gaseous substance in the atmosphere above the liquid or solid with which it is in equilibrium.

Vapor pressure depression—The decrease in the vapor pressure of a liquid caused by the presence of dissolved solute; a colligative property.

Vaporization—The transformation of a liquid into a gas.

Water dissociation constant (Kw)—The equilibrium constant of the water dissociation reaction at a given temperature; equal to 10–14 at 25°C (298 K).